1. Field of the Invention
The present invention relates to a surface acoustic wave filter in which IDTs (Inter Digital Transducer) on an input side and on an output side each made of a plurality of electrode digits are formed on a piezoelectric substrate with a spacing interposed therebetween.
2. Description of the Related Art
FIG. 1 is a plan view showing an example of a conventional surface acoustic wave filter. This surface acoustic wave filter 102 has IDTs 106, 108 on an input side and on an output side, which are surface acoustic waves transducers, formed with a spacing interposed therebetween on piezoelectric substrate 104 made of crystal and the like. The width of each electrode digit of each of IDTs 106, 108 is .lambda./4 where .lambda. is the wavelength of the surface acoustic wave during resonance. One electrode 114 of IDTs 106, 108 are connected to input terminal 116 and output terminal 118 the other electrodes 120 are both grounded.
In this surface acoustic wave filter 102, when an alternating signal is supplied from input terminal 116, IDT 106 causes a surface acoustic wave to be excited on piezoelectric substrate 104. The wave is propagated in a direction orthogonal to the extending direction of each electrode digit 112 constituting IDT 106, forms a standing wave while repeating multiple reflection between IDTs 106 and 108, and is received in IDT 108 and then outputted from output terminal 118 as an electric signal.
Surface acoustic wave filter 102 having such construction as described above is also referred to as a vertical mode resonator type surface acoustic wave filter since the surface acoustic wave is propagated in the vertical direction and the resonance of the modes distributed in the same direction as the propagated direction is utilized.
FIG. 2 is a graph showing the frequency characteristic of surface acoustic wave filter 102. In this figure, the horizontal axis represents the frequency and the vertical axis represents the attenuation factor, respectively.
An ideal characteristic for surface acoustic wave filter 102 which is a band-pass filter is to pass signals only in the pass-band near the center frequency f. However, signals are actually passed in some degree even at a point away from the center frequency, which phenomenon appears as a spurious signal in the frequency characteristic.
Particularly, in surface acoustic wave filter 102 which is a vertical mode resonator type surface acoustic wave filter, a typical spurious signal 122 occurs in a region higher than the center frequency, which causes a reduction in the amount of attenuation out of the band and a deterioration in the frequency characteristic as a band-pass filter.